A joining structure, as shown in FIG. 22 or 23, has been conventionally used for a join, for instance, between a steel base structure and a foundation. The conventional joining structure is formed by welding a bolt connection base plate 11 to an end of a structural member 10 and reinforcing the joining structure with reinforcing ribs 12 attached between the structural member 10 and the base plate 11. The reinforcing ribs 12 are tabular members extending in the direction of the principal stress of the structural member 10 and are attached to protrude from the surface of the structural member 10 in the shape of a T.
In a conventional joining structure as described above, however, there is a problem that, when a bending moment is applied on the structural member 10, a large out-of-plane bending stress concentrates at the portions of the structural member 10 near the toes of the reinforcing ribs 12 and, as a consequence, the performance of the structure is deteriorated. Another problem is that, when the reinforcing ribs 12 are welded to the structural member 10, structural defects are likely to occur in the boxing welding portions at the upper ends of the reinforcing ribs 12 as a result of the combined effects of the residual stress caused by welding heat and the material degradation of the heat affected zones at the weld toes, causing the proof stress and the fatigue property to deteriorate. Those problems are common to many types of joining structures in which reinforcing ribs 12 are welded to structural members 10 in the form of T-joints and, in view of this, the Japanese Society of Steel Construction points out, in “Guideline for Fatigue Design of Steel Structures and Its Interpretation”, that a join in which a gusset is welded by fillet welding or groove welding adversely affects the proof stress and fatigue property of a steel member and, therefore, attention has to be paid to this in the design of structures.